In these years, demand for large-capacity optical discs has been growing remarkably. Examples of the technique to increase the recording capacity of optical discs include a method which adopts shorter wavelengths for the recording/reproducing beam, a method which adopts a higher NA (number of aperture) for the objective lens in the recording/reproducing beam irradiation optical system, and a method which adopts multiple layers for the recording layer. Among these, the adoption of the multiple layers for the recording layer enables the optical disc to have a large capacity in a lower cost than the adoption of the shorter wavelengths or the adoption of the higher NA.
For example, a DVD or HD-DVD disc having two recording layers adopts a structure in which the two recording layers are laminated together with the intermediary of a transparent resin intermediate layer. Specifically, the structure as viewed from the incident side of the recording/reproducing beam is such that a first 0.6-mm transparent resin substrate, a first recording layer, a first translucent reflecting layer, the transparent resin intermediate layer, a second recording layer, a second reflecting layer, an adhesive layer and a second 0.6-mm transparent resin substrate are laminated in this order; wherein the transparent resin intermediate layer is formed as follows: an ultraviolet-curable resin composition is applied to the first translucent reflecting layer, pressed against a transparent resin stamper having an asperity pattern including the guide grooves for guiding the recording/reproducing beam and cured with ultraviolet light, and thereafter the stamper is released and thus the asperity pattern is transferred to the surface of the cured product to form the transparent resin intermediate layer.
A BLU-RAY optical disc having two recording layers has a structure as viewed from the incident side of the recording/reproducing beam is such that a 0.1-mm light transmissive layer, a first recording layer, a first translucent reflecting layer, a transparent resin intermediate layer, a second recording layer, a second reflecting layer and a 1.1-mm resin substrate are laminated in this order; wherein the transparent resin intermediate layer is formed as follows: an ultraviolet-curable resin composition is applied to the second recording layer, pressed against a transparent resin stamper having an asperity pattern including the guide grooves for guiding the recording/reproducing beam and cured with ultraviolet light, and thereafter the stamper is released and thus the asperity pattern is transferred to the surface of the cured product to form the transparent resin intermediate layer.
In general, the above-described transfer method is referred to as the 2P (Photo Polymerization) method, and the ultraviolet-curable resin composition used in the 2P method is referred to as the 2P resin.
Examples of the resin for the transparent resin stamper include acrylic resin, methacrylic resin, polycarbonate resin, polyolefin resin (in particular, amorphous polyolefin), polyester resin, polystyrene resin and epoxy resin. From the viewpoints of the properties of the 2P resin after curing such as the mold-release performance, the low hygroscopic property and the shape stability, preferable among these is amorphous polyolefin; from the viewpoint of the material cost, preferable among these is polycarbonate resin.
In general, polycarbonate resin is used for the 0.6-mm resin substrate in a DVD or HD-DVD disc and for the 1.1-mm resin substrate in a BLU-RAY optical disc. When polycarbonate resin is used as the transparent resin stamper, the application of an ultraviolet-curable resin other than the 2P resin to the resin substrate side facilitates the release of the resin stamper made of polycarbonate from the 2P resin layer having been cured.
The properties required for the 2P resin are as follows:
(i) Satisfactory mold-release performance from the transparent resin stamper;
(ii) Satisfactory transfer performance from the transparent resin stamper;
(iii) Small warping of the disc substrate after ultraviolet curing;
(iv) No deformation of the transferred asperity pattern under high temperature and high humidity; and
(v) Particularly in an optical disc having a recording layer made of a dye, no diffusion of the dye into the transparent resin intermediate layer under high temperature and high humidity.
Poor mold-release performance from the transparent resin stamper partially exfoliates the transparent resin intermediate layer together with the transparent resin stamper to cause defects. Poor transfer performance causes errors at the time of recording/reproducing. Large warping of the disc substrate after ultraviolet curing causes the following problems: the recording layer or the reflecting layer cannot be uniformly formed on the formed asperity pattern; lamination of the second substrate is disturbed in the case of a DVD or HD-DVD disc; and uniform formation of the 0.1-mm light transmissive layer is disturbed in the case of the BLU-RAY optical disc. Moreover, the deformation of the asperity pattern under high temperature and high humidity or the diffusion of the dye into the transparent resin intermediate layer leads to nonequivalence in recording properties (for example, jitter properties) between the first and second recording layers.
The 2P resin described in any of Patent Documents 1 to 4 undergoes the asperity pattern formation on a glass substrate with a metal stamper; none of these Patent Documents describes the asperity pattern formation with a transparent resin stamper. Additionally, Patent Documents 5 to 8 each include a description of the 2P resin without including any description of the resin according to the present invention.    Patent Document 1: JP-A-5-59139    Patent Document 2: JP-A-5-132534    Patent Document 3: JP-A-5-140254    Patent Document 4: JP-A-5-132506    Patent Document 5: JP-A-2003-331463    Patent Document 6: JP-A-2004-288242    Patent Document 7: JP-A-2004-288264    Patent Document 8: JP-A-2005-332564